1. Field of the Invention
This invention relates to a light-emitting diode (hereinafter referred to as LED) and a light-emitting device employing a semiconductor light-emitting element such as a semiconductor laser diode (hereinafter referred to as LD).
2. Description of the Related Art
In recent years, much attention has been paid to so-called white LEDs, where white light is emitted using a single chip comprising a combination of a blue LED employed as an excitation light source and a yellow fluorescent substance such as YAG:Ce. Although this white LED is simple in construction, the white color to be emitted therefrom is poor in a red component. Because of this, the white light obtained from a combination consisting of only the blue emission to be derived from an excitation light source and the yellow fluorescent substance has been realized as being defective in that it lacks in color rendering. Therefore, this white light is accompanied with a problem that it is difficult to apply it for ordinary illumination or as a backlight for a liquid crystal display where high color rendering is demanded.
This problem may be overcome by the co-use of a red fluorescent substance. For example, it is possible to obtain an emission color which is excellent in color reproducibility using a combination of a green fluorescent substance and a red fluorescent substance with a blue LED. As the green fluorescent substance, it is possible to employ (Sr, Ca, Ba)(Al, Ga)2S4:Eu, BaMgAl10O17:Eu, etc. As the red fluorescent substance, it is possible to employ (Ca, Sr)S:Eu, CaLa2S4:Ce, etc.
There is also proposed a light-emitting device in JP-A 2000-509912 (KOHYO) wherein a blue fluorescent substance, a green fluorescent substance and a red fluorescent substance are used in combination with an ultraviolet LED. Since the color of emission to be obtained depends only on the mixing ratio of fluorescent substances, the preparation of the light-emitting device would appear to be simple. Therefore, as far as the color rendering is concerned, this light-emitting device is more excellent than the light-emitting device to be obtained from a blue LED which is combined with only a yellow fluorescent substance.
When several kinds of fluorescent substances are employed, there is the possibility that fluorescence may be re-absorbed. There is also proposed a light-emitting device in JP-A 2005-277127 (KOKAI) wherein a plurality of fluorescent substances each differing in fluorescent wavelength emitted are disposed along the route of light to be emitted externally from an excitation element in such an order that the fluorescent substance exhibiting a longer fluorescent wavelength is arranged more close to the excitation element, thereby suppressing the re-absorption of fluorescence. Specifically, a layer of fluorescent substance emitting a long wavelength (a wavelength closer to red color) is disposed in the vicinity of the excitation element and a layer of fluorescent substance emitting a short wavelength (a wavelength closer to blue color) is disposed at an outer position located away from the excitation element, thereby suppressing the re-absorption of fluorescence and enhancing the brightness of the light.